The power required to drive a helicopter tail rotor is typically delivered through a long, segmented drive shaft. The drive shaft is segmented to permit operation below critical vibration modes associated with rotary winged flight. Each segment must be rotatively supported by the aircraft airframe, and must be coupled to the next segment by means of an alignment-forgiving coupling to accommodate airframe deflections. In addition, the drive shaft must be lightweight, reliable and fail safe.
Typically, a tail rotor drive shaft assembly, which receives input from a main rotor gearbox and drives an intermediate gearbox located along the aft portion of and supported by the tail boom of the aircraft, comprises a plurality of shaft segments supported by rolling element bearing assemblies and coupled via alignment-forgiving torque drive assemblies. The rolling element bearing assemblies typically comprise a conventional ball bearing within a rubber bladder filled with viscous silicon fluid interposed between and isolating the outer race of the ball bearing from the airframe structure. The alignment-forgiving torque drive couplings typically comprise a series of thin disc members stacked together and bolted at three angular positions along a face surface of an end flange of one shaft segment and at three interspaced positions of another shaft segment. Field problems include cracking of the couplings, loss of the viscous damping fluid and seizure of the bearing assemblies. Bearing seizure is associated with friction contact of the ball bearing outer race with the airframe structure which results in an extreme overtemperature condition, having the potential for developing an inertia weld and loss of drive in the tail system. In addition, the bearing support and coupling assemblies require frequent and painstaking inspection associated with preventive maintenance. To complicate matters, the location of the drive shaft is not favorable for maintenance. All of these characteristics have been compounded by the fact that helicopters are operating at higher rotor RPM than ever before.
In U.S. Pat. No. 4,560,364 there is disclosed a flexible shaft coupling in which the primary, alignment-forgiving coupling comprises two spaced apart thin metal disks which are bonded together at their outer periphery. A backup coupling, which engages automatically only if the primary coupling fails, consists of a loose plug and socket arrangement. The male portion is significantly smaller than the female portion within which it will engage. Should the primary coupling fail, the unsupported shaft will have a degree of looseness, after torque to the rotor has been removed, that provides the ground crew observers a clue that the primary coupling has failed. However, this requires gaining access to the shaft for visual observation, and requires constant post-flight observation to detect when the primary coupling has failed.